Traditionally, when a phased array antenna is tested using a planar nearfield scanner, RF equipment is used to receive RF from an Array Under Test (“AUT”) or transmit RF to the AUT. In order to establish a concise reference point for obtaining the key phase and amplitude deltas for determining AUT performance, the equipment is configured in such a manner that establishes a reference point at the nearfield probe and a reference point at the AUT. Typically these points are established by placing a mixer in line and using a local oscillator to phase lock the points together. During nearfield scanning measurements, data is acquired via a RF receiver where amplitude and phase measurements are determined by the delta between the reference and test points. After the entire active aperture has been scanned and data collected, the data is post processed via a Fourier Transform to obtain a pattern of the farfield energy in visible space. This method works for phased arrays that use analog beamforming.
As phased arrays become more advanced, analog beamforming has evolved into digital beamforming, that is the traditional analog beamforming networks have been replaced by digital equipment. Analog receivers and exciters have been replaced by digital receivers and exciters which greatly improve key phased array performance parameters such as signal to noise, beamforming error elimination, and clutter attenuation aided by decorrelation.
The change to digital beamforming, however, has introduced new challenges in the nearfield test methodology. When a digital array transmits to the nearfield scanner probe, a digital word is translated by the exciter digital-to-analog converter and transmitted as RF to the nearfield scanner probe. When the digital array receives from the nearfield scanner probe, an analog-to-digital converter on the digital receiver converts the analog data to digital data comprising in-phase and quadrature-phase (I/Q) data components. This I/Q data stream is then bussed to various data processing locations within the phased array radar system. In existing nearfields, the nearfield scanner probe uses an analog receiver and exciter (RxEx). The existing nearfields thus have the cumbersome problem of having to correlate digital I/Q data from the digital array with analog RF data of the nearfield scanner probe.
This disclosure describes a new method of scanning a digital beamformed phased array in a nearfield by creating an open architecture approach to replacing the analog reference points with digital reference points and the ability to perform measurements.